In hazardous environments such as those where toxic chemicals or vapours exist or where nuclear, biological and chemical (NBC) warfare agents are present or may be impending, personnel must wear protective clothing in addition to, or integral with, their regular clothing and equipment in order to protect themselves from these perilous elements. For personnel such as aircrew, who require helmets or other headgear, such protective clothing must not impair the intended functions of the headgear. For instance, the helmet that is worn by aircrew has several functions that must be maintained, even during NBC operations. These functions include protection of the head in the event of a crash, protection of the ear against excessive aircraft noise, and provision of means for electronic communications, to name a few. In most instances, the headgear or helmet is very tight fitting and the smallest protrusion or pressure point can cause considerable discomfort after only a few minutes of wear. If a protective hood is to be worn under the headgear, then it must not cause discomfort to the wearer and must maintain the functions of the headgear (i.e. hearing protection and communications intelligibility) to a specified level.
There are only a few under-the-helmet NBC respirators intended for aircrew use, each of which utilizes impermeable rubber or other rubberized materials. Impermeable rubberized materials are not always suitable for use in an NBC protection system as they create heat stress which is a major concern in the design of modern NBC equipment. The current breathable NBC garments are thick and bulky, and are generally only acceptable because nothing else is available.